1. Technical Field
This invention relates generally to audio amplifiers and, more particularly, to a circuit and method for generating a clipping distortion detect output signal for an integrated circuit audio amplifier which has an adjustable dependence on supply voltage.
2. Discussion
It is generally common for audio amplifiers to produce audio outputs which can become distorted due to clipping. Clipping is a well known phenomenon which often occurs where the audio amplifier output exceeds the amplifier's dynamic range. As a consequence, the output is no longer able to follow the dynamics of the input in a linear manner, and the output signal is clipped. This causes the expected amplifier peak-to-peak voltage output to be reduced, thereby resulting in an amount of distortion. To handle the distortion problem, a number of currently existing audio amplifiers generate a clipping distortion detect signal which indicates that the audio outputs are becoming distorted due to clipping. With the clipping distortion detect signal, audio amplifier systems are usually capable of responding to the detect signal to control system operations so as to bring the amplifier output to within an acceptable amount of distortion.
In the past, conventional integrated circuit audio amplifiers have employed clipping distortion detect schemes which include a comparator circuit that compares the amplifier input signal to the amplifier output signal divided by the gain of the amplifier. When the absolute value of the input signal is greater than the absolute value of the output signal divided by the voltage gain, plus some fixed amount of offset, the comparator fires. The comparator thereby generates a clipping distortion detect output signal which indicates that the amplifier output signal is distorted. With this approach, the fixed amount of offset is directly related to the level of distortion that is required to fire the comparator.
Audio amplifier feedback systems which utilize the above conventional clipping distortion indicator circuit generally set the maximum level of distortion that will be heard by a listener. When this amount of distortion is exceeded, the clipping distortion detect output signal is generally used to cause the audio amplifier system to adapt to the excessive distortion. This usually involves triggering a reduction in the amplifier input or an increase in the amplifier supply voltage in order to reduce the amount of distortion so as to bring the distortion to within an acceptable amount.
As previously mentioned, the output dynamic range of the amplifier is typically dependent on amplifier supply voltage. In a number of systems, the supply voltage can vary, especially for automotive applications where a normal radio operating range may vary between 9 and 18 Volts, for example. Since the amount of clipping distortion is dependent on the available output voltage swing, the amount of clipping will fluctuate with changes in the supply voltage. One approach for an output sensing circuit is described in U.S. Pat. No. 5,068,620, issued to Botti et al, which generates a signal indicative of output distortion that is independent of power supply voltage. The aforementioned prior art circuit employs a pair of comparators for comparing a reference voltage with the audio amplifier output divided by the gain. To achieve supply independence, the Botti et al approach obtains the reference voltage as a divided portion of the input voltage.
In addition, conventional clipping distortion indicator outputs have been found to be dependent upon operating temperature. Generally, supply voltage and temperature dependencies have been observed as a change in the amount of audio signal clipping that is required to activate the clipping distortion indicator as the operating temperature or supply voltage is varied. This is typically measured in percent of waveform clipped or percent total harmonic distortion (THD). Most currently existing audio amplifier systems tend to achieve a percent total harmonic distortion that varies with the supply voltage and temperature. Conventional systems generally do not provide for adjustable compensation due to the effects caused by the operating temperature and/or supply voltage and are therefore unable to achieve an optimal constant maximum distortion level.
Further, for some audio amplifier applications it would be desirable to achieve a distortion indicator signal that has an adjustable dependence upon the amplifier supply voltage. For example, a battery powered audio amplifier system will generally, over the life of the battery, be subjected to a decrease in the amount of energy that is available from the battery. This is because the battery will drain with use over time and thereby generally result in a reduced supply voltage. Since the maximum output power for an audio amplifier is directly related to the maximum limitation on the clipping induced distortion, a constant distortion level may cause the maximum output power to decay to an unacceptable level at low battery voltages. For some applications, during low voltage battery operation, it may be optimal to trade off an increase in maximum distortion level for a higher output power.
It is therefore one object of the present invention to provide for a circuit and method of generating a distortion detect output signal for an integrated circuit audio amplifier.
It is another object of the present invention to provide for a circuit and method of generating the distortion detect output signal for an audio amplifier that can be independent of the amplifier supply voltage.
It is a further object of the present invention to provide for such a circuit and method for generating the distortion detect output signal for an audio amplifier system that can be independent of the supply voltage and/or operating temperature, and that may be employed to achieve a constant maximum distortion level.
It is yet a further object of the present invention to provide for a circuit and method for generating a distortion detect output signal for an audio amplifier system which allows for an adjustable supply dependence so as to achieve an increased maximum distortion level at low supply voltages than at higher supply voltages.